Non-magnetic austenitic stainless cast steel and manufacturing method of the same

ABSTRACT

A non-magnetic austenitic stainless cast steel having high yield strength at room temperature without losing ductility at liquid nitrogen temperature, permitting easy casting without requiring the solution heat treatment, containing (by mass %) 0.08 percent or less carbon, 0.1 to 1.5 percent silicon, 0.1 to 1.5 percent manganese, 13 to 15 percent nickel, 18 to 19 percent chromium, 2 to 2.5 percent molybdenum, 0.005 to 0.1 percent aluminum, and 0.12 to 0.2 percent nitrogen, with the remainder of the steel being iron and incidental impurities, and having an elongation at liquid nitrogen temperature having 30 percent or more, 0.2% yield strength at room temperature having 240 MPa or more and relative magnetic permeability having 1.10 or less.

FIELD OF THE INVENTION

[0001] The present invention relates to a non-magnetic austenitic stainless cast steel for a member for an electromagnet used below liquid nitrogen temperature and to a method of manufacturing such a non-magnetic austenitic stainless cast steel.

DESCRIPTION OF THE RELATED ART

[0002] Recently, techniques have been developed rapidly that relate to nuclear fusion reactors or the like using large magnets. Many of these techniques conventionally employ the superconductivity, and because of increase of the cost particularly for cooling of the coil, techniques of achieving cost reduction have been developed. Thus, demands are increased for members for electromagnets used at liquid nitrogen temperature (77K).

[0003] Such members are required to be in excellent in strength and ductility at low and room temperatures as for superconducting material, and should have non-magnetic properties.

[0004] As materials to cope with above requirements, there are proposed SUS304LN stainless steels such as 18% Cr-8% Ni which are nitrogen-rich austenitic stainless steels and nitrogen-Mn-rich austenite steels as described in Japanese Laid-Open Patent Publication S55-51432.

[0005] Since the aforementioned materials are steel products subjected to processing such as hot roll and are poor in hot workability, for example, Japanese Laid-Open Patent Publication H07-90366 discloses a technique of improving the hot workability. However, any technique has not been disclosed that configures a desired form by casting.

[0006] Conventional nitrogen-rich austenitic stainless steel for low temperature is manufactured by such a steel manufacturing method that decreases the carbon content to a minimum, and the carbon is to be soluted in the matrix to ensure the ductility at low temperature. Thus the solution heat treatment is required in which the steel is heated above 1150° C. and then rapidly cooled.

[0007] For example, with respect to members used for electromagnets, since the shape is complicated in three dimensions, it is required to manufacture products by casting from the viewpoint of cost reduction. However, when the solution heat treatment is processed on a member, deformation due to its own weight is inevitable in heating because of a shortage of thermal strength, while it is difficult to avoid deformation due to large stress caused by cooling.

[0008] As a result of dedicated studies in view of the aforementioned problems, a technique was found out of providing non-magnetic austenitic stainless cast steel having a high yield strength at room temperature without losing ductility at liquid nitrogen temperature, and permitting easy casting without requiring the solution heat treatment.

DISCLOSURE OF THE INVENTION

[0009] To solve the above problems, a first embodiment of the present invention is a non-magnetic austenitic stainless cast steel comprising (by mass %) 0.08 percent or less carbon, 0.1 to 1.5 percent silicon, 0.1 to 1.5 percent manganese, 13 to 15 percent nickel, 18 to 19 percent chromium, 2 to 2.5 percent molybdenum, 0.005 to 0.1 percent aluminum, and 0.12 to 0.2 percent nitrogen, with the remainder of the steel being iron and incidental impurities, and having an elongation at liquid nitrogen temperature having 30 percent or more, 0.2% yield strength at room temperature having 240 MPa or more.

[0010] A second embodiment of the present invention is a non-magnetic austenitic stainless cast steel, wherein the steel comprises a relative magnetic permeability having 1.10 or less.

[0011] A third embodiment of the present invention is the non-magnetic austenitic stainless cast steel obtained by being heated at temperature T(° C.) ranging from 300° C. to 550° C. and meeting equations (1) and (2) described below for two hours or more and subsequently cooled to below 200° C. by air-cooling or slow-cooling:

T≦12500[N]−1200  Eq.1

T≦−5000[N]+1300  Eq.2

[0012] wherein [N] indicates the nitrogen content (%).

[0013] A fourth embodiment of the present invention is the non-magnetic austenitic stainless cast steel that is a member used below liquid nitrogen temperature.

[0014] A fifth embodiment of the present invention is the non-magnetic austenitic stainless cast steel that is a member for an electromagnet.

[0015] A sixth embodiment of the present invention is a method of manufacturing a non-magnetic austenitic stainless cast steel of preparing a cast steel comprising (by mass %) 0.08 percent or less carbon, 0.1 to 1.5 percent silicon, 0.1 to 1.5 percent manganese, 13 to 15 percent nickel, 18 to 19 percent chromium, 2 to 2.5 percent molybdenum, 0.005 to 0.1 percent aluminum, and 0.12 to 0.2 percent nitrogen, with the remainder of the steel being iron and incidental impurities, heating the cast steel at temperature T(° C.) ranging from 300° C. to 550° C. and meeting equations (1) and (2) described below for two hours or more, and subsequently cooling the cast steel to below 200° C. by air-cooling or slow-cooling:

T≦12500[N]−1200  Eq.1

T≦−5000[N]+1300  Eq.2

[0016] wherein [N] indicates the nitrogen content (%).

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a schematic view showing the range of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Embodiments of the present invention will be described below. The present invention relates to a non-magnetic austenitic stainless cast steel that is a member used below liquid nitrogen temperature, for example, a member for an electromagnet, and to a method of manufacturing such a non-magnetic austenitic stainless cast steel.

[0019] As described previously, in austenitic stainless cast steels, the solution heat treatment thereon ensures ductility at low temperature and strength at room temperature. However, when the steels are manufactured by casting, since deformation may occur as in the case of performing the solution heat treatment, it is necessary to avoid heating at high temperature and rapid cooling. In order to overcome such matters, it is required to first select specific component atoms, and control the selected component atoms in optimal content ranges. It is further required to employ a heat treatment method corresponding to the content ranges of the component atoms.

[0020] An austenitic stainless cast steel of the present invention contains (by mass %) 0.08 percent or less carbon, 0.1 to 1.5 percent silicon, 0.1 to 1.5 percent manganese, 13 to 15 percent nickel, 18 to 19 percent chromium, 2 to 2.5 percent molybdenum, 0.005 to 0.1 percent aluminum, and 0.12 to 0.2 percent nitrogen, with the remainder of the steel being iron and incidental impurities. Reasons for chemical compositions in the present invention will be described below.

[0021] C: 0.08 % or less; C atoms are effective to ensure non-magnetic properties due to stabilization of austenite. Carbon of more than 0.08 percent combines with chromium easily between dendrites, is precipitated as chromium carbide and thereby impairs the ductility. Accordingly, the carbon content is made 0.08 percent or less. In addition, carbon of less than 0.03 percent is not effective. Therefore, the carbon content is preferably in a range from 0.03 percent to 0.08 percent.

[0022] Si: 0.1 to 1.5 %; Si atoms are contained in steel as deoxidizing atoms. Silicon of less than 0.1 percent is not sufficient in deoxidizing effect and tends to cause defects in the casting, while silicon of more than 1.5 percent tends to cause casting blockage. Accordingly, the silicone content ranges from 0.1 percent to 1.5 percent.

[0023] Mn: 0.1 to 1.5 %; Mn atoms are contained in steel as deoxidizing atoms, as well as Si. Manganese of less than 0.1 percent is not sufficient in deoxidizing effect and tends to cause defects in the casting, while manganese of more than 1.5 percent saturates the deoxidizing effect. Accordingly, the manganese content ranges from 0.1 percent to 1.5 percent.

[0024] Ni: 13 to 15%; It is well known Ni and Cr atoms are inevitable for austenitic steel, and different structures are obtained depending on their contents. Ni atoms are effective in suppression of δ-ferrite developing in the solidification process, and stabilize the austenite structure. However, nickel of less than 13 percent is less effective, while nickel of more than 15 percent causes 0.2% yield strength at room temperature to decrease. Accordingly, the nickel content ranges from 13 percent to 15 percent.

[0025] Cr: 18 to 19 %; Cr atoms are inevitable to provide steel with an austenitic structure. Chromium of less than 18 percent leads to a shortage of the yield strength at room temperature, while chromium of more than 19 percent tends to form a ferrite structure and does not permit to achieve non-magnetic properties. Accordingly, the chromium content ranges from 18 percent to 19 percent.

[0026] Mo: 2 to 2.5%; Mo atoms contribute to improvements in ductility. Molybdenum of less than 2 percent exhibits less effects, while molybdenum of more than 2.5 percent tends to form a ferrite structure and does not permit to achieve non-magnetic properties, as well as Chromium. Accordingly, the molybdenum content ranges from 2 percent to 2.5 percent.

[0027] Al: 0.005 to 0.1%; Al atoms are contained in steel as deoxidizing atoms. Aluminum of less than 0.005 percent fails to deoxidize sufficiently and tends to cause defects in the casting. Further, aluminum of more than 0.1 percent tends to form AlN in the case of nitrogen contained steel. Accordingly, the aluminum content ranges from 0.005 percent to 0.1 percent.

[0028] N: 0.12 to 0.2 %; N atoms are alloy atoms effective to increase the yield strength at room temperature. Meanwhile, the excessive nitrogen content impairs the ductility at liquid nitrogen temperature, and thus N atoms are important. Nitrogen of less than 0.12 percent makes it difficult to obtain the yield strength at room temperature having 240 MPa or more, while nitrogen of more than 0.2 percent impairs the ductility at liquid nitrogen temperature. Accordingly, the nitrogen content ranges from 0.12 percent to 0.2 percent.

[0029] In the present invention, the non-magnetic austenitic stainless cast steel is obtained by heating a cast steel at temperature T(° C) ranging from 300° C. to 550° C. and meeting equations (1) and (2) described below for two hours or more, and subsequently cooling the cast steel to below 200° C. by air-cooling or slow-cooling:

T≦12500[N]−1200  Eq.1

T≦−5000[N]+1300  Eq.2

[0030] wherein [N] indicates the nitrogen content (%).

[0031] As a heat treatment condition, the steel is heated and held at a heating temperature in a range from 300° C. to 550° C. for two hours or more to relax the casting residual stress, and then cooled to below 200° C. by air-cooling or slow-cooling.

[0032] The reasons for the heating temperature raging from 300° C. to 550° C. will be described below. The heating temperature of less than 300° C. requires a long time to relax the residual stress, and is not practical. The heating temperature exceeding 550° C. causes nitrogen atoms soluted in the matrix to react with other atoms and to form precipitation, and as a result, decreases the ductility at liquid nitrogen temperature.

[0033] Further, the heating time of the heat treatment condition is two hours or more, the reason for which is that the heating time of less than two hours sometimes causes the entire casting of large size not to reach a predetermined heat treatment temperature.

[0034] Furthermore, the reason for employing an air-cooling or a slow-cooling as the cooling rate to below 200° C. is that faster rates than this cooling rate deform products due to distortions caused by cooling.

[0035] The cooling covers a cooling rate from 25° C. to 100° C./hr dependent on the size of cast and the heating furnace being used.

[0036] The air-cooling means a cooling rate in the air, which covers a cooling rate less than about 200° C./hr.

[0037] The slow-cooling means a cooling in the heating furnace, which covers a cooling rate from 25 to 100° C./hr.

[0038] Moreover, as a result of repeated experiments, it was found out experimentally that meeting following equations (1) and (2) is important in the relationship between heat treatment temperature T(° C.) and nitrogen content [N] (%).

T≦12500[N]−1200  (1)

T≦−5000[N]+1300  (2)

[0039] In other words, heat treatments at heating temperatures out of the range meeting equations (1) and (2) result in 0.2% yield strength of less than 240 MPa or elongation at liquid nitrogen temperature of less than 30 percent.

[0040] In the present invention, the non-magnetic austenitic stainless cast steel is a member used below liquid nitrogen temperature, and can be used for a member for an electromagnet. For example, the steel can be used as a coil form for an electromagnet.

[0041] Further, in the present invention, the non-magnetic austenitic stainless cast steel is manufactured by preparing a cast steel containing (by mass %) 0.08 or less percent carbon, 0.1 to 1.5 percent silicon, 0.1 to 1.5 percent manganese, 13 to 15 percent nickel, 18 to 19 percent chromium, 2 to 2.5 percent molybdenum, 0.005 to 0.1 percent aluminum, and 0.12 to 0.2 percent nitrogen, with the remainder of the steel being iron and incidental impurities, heating the cast steel at temperature T(° C.) ranging from 300° C. to 550° C. and meeting equations (1) and (2) described below for two hours or more, and subsequently cooling the case steel to below 200° C. by air-cooling or slow-cooing:

T≦12500[N]−1200  Eq.1

T≦−5000[N]+1300  Eq.2

[0042] wherein [N] indicates the nitrogen content (%).

[0043] In other words, the cast steel to have a composition described above is prepared, is heated for two hours or more at temperature T (° C.) indicated by T≦12500[N]−1200 and T≦−5000[N]+1300 in a range from 300° C. to 550° C. , and is cooled to below 200° C. by air-cooling or slow-cooling, thereby manufacturing the austenitic stainless cast steel.

EXAMPLES

[0044] The present invention will be described specifically below based on examples.

[0045] Samples prepared to have compositions shown in Table 1 are melted using a 30kVA high-frequency furnace, and are cast to JIS G5122B test specimens (hereinafter referred to as B specimens) using a CO₂-silica mold. The B specimens are maintained at a heating temperature in a range of 300° C. to 550° C. for two hours, subsequently cooled to below 200° C. by air-cooling or slow-cooling, and formed into ASTM G-tensile test specimens (with a parallel portion diameter of 6 mm).

[0046] The ASTM G-tensile test specimens are subjected to tensile tests at room temperature and liquid nitrogen temperature (77K). On the test specimens after the tensile test at room temperature, the relative magnetic permeability of a break portion is measured. Specimens with relative magnetic permeability having 1.10 or less were judged as a non-magnetic material. The heating temperature, cooling rate, numerals of equations (1) and (2) of the present invention obtained by using the nitrogen content and heating temperature shown in Table 1, and measurement values are shown in Table 2. TABLE 1 List of Compositions Composition (mass %) Re- No. C Si Mn Ni Cr Mo Al N marks 1 0.05 1.03 0.88 14.11 18.20 2.25 0.017 0.153 Exam- 2 0.06 1.02 1.00 14.03 18.44 2.11 0.066 0.162 ple of 3 0.06 1.04 1.02 14.07 18.93 2.10 0.034 0.179 the in- 4 0.06 1.05 1.33 14.92 18.22 2.04 0.078 0.191 vention 5 0.03 1.20 1.03 14.55 18.55 2.20 0.062 0.136 6 0.05 0.99 1.22 14.67 18.33 2.28 0.053 0.121 7 0.04 1.10 0.99 13.88 18.03 2.42 0.045 0.183 8 0.05 0.88 1.20 13.12 18.55 2.12 0.054 0.157 9 0.03 0.77 1.11 14.22 18.83 2.25 0.082 0.133 10 0.04 1.25 1.34 13.90 17.18 2.44 0.046 0.083 Com- 11 0.06 0.90 0.92 13.97 18.35 2.19 0.048 0.124 parative 12 0.05 1.07 0.90 12.54 20.25 2.29 0.006 0.024 exam- 13 0.05 1.06 0.87 13.35 18.75 2.34 0.010 0.123 ple 14 0.05 0.83 1.08 16.02 19.30 2.62 0.020 0.285 15 0.04 1.10 1.00 13.21 18.60 2.25 0.052 0.170 16 0.03 0.95 0.98 14.15 18.12 2.03 0.040 0.192 17 0.05 1.12 0.88 14.33 18.50 2.40 0.050 0.143

[0047] TABLE 2 List of heating temperatures and measurement results Heating 0.2% yield Elongation Relative temperature Cooling T of Eq. 1 T of Eq. 2 strength at 77 K magnetic No. (° C.) rate (° C.) (° C.) (MPa) (%) permeability Remarks 1 400 air cooling 712.5 535.0 243 33 1.01 or less Example of 2 475 825.0 490.0 242 31 1.01 or less the invention 3 320 1037.5 405.0 250 32 1.01 or less 4 310 1187.5 345.0 260 31 1.01 or less 5 545 500.0 620.0 245 34 From 1.01 to 1.02 6 300 312.5 695.0 242 36 1.01 or less 7 350 1087.5 385.0 255 32 1.01 or less 8 325 762.5 515.0 255 32 From 1.01 to 1.02 9 400 slow-cooling 462.5 635.0 250 33 1.01 or less 10 400 air cooling −162.5 885.0 198 55 From 1.01 to 1.02 Comparative 11 400 350.0 680.0 230 34 1.01 or less Example 12 545 −900.0 1180.0 228 21 From 1.01 to 1.02 13 545 337.5 685.0 252 5 From 1.01 to 1.02 14 350 2362.5 −125.0 235 14 From 1.01 to 1.02 15 500 925.0 450.0 235 31 1.01 or less 16 400 1200.0 340.0 250 18 1.01 or less 17 575 587.5 585.0 245 25 1.01 or less

[0048] In Table 2, Nos. 1 to 9 are examples of the present invention, while Nos. 10 to 16 are comparative examples out of the scope of the present invention. As can be seen from Table 2, the examples of the present invention have 0.2% yield strength at room temperature having 240 MPa or more and elongation at liquid nitrogen temperature having 30 percent or more, and are thus excellent in ductility. Further the examples have the relative magnetic permeability having 1.10 or less, and are thus suitable for a member for an electromagnet.

[0049] Among the comparative examples, Nos. 10, 12 and 14 have compositions out of the range of the present invention, and Nos. 10 to 16 underwent the heat treatment at higher temperatures than values of T of equations (1) and (2). No. 17 underwent the heat treatment at a temperature out of the range of the present invention. Therefore, either or both of values (MPa) of 0.2% yield strength and values (%) of elongation at liquid nitrogen temperature indicate low values.

[0050] Further, FIG. 1 shows the relationship between the nitrogen content and heating temperature on the examples of the present invention and comparative examples. In FIG. 1, a region within the range surrounded by solid lines indicative of 300° C. and 550° C. and by solid lines of equations (1) and (2) of the present invention corresponds to the present invention. Circles in the figure indicate the examples of the present invention, while crosses in the figure indicate the comparative examples.

[0051] In other words, the examples of the present invention meet the heat treatment condition that the heating temperature is in a range from 300° C. and 550° C. and meets equations of T≦12500[N]−1200 and T≦−5000[N]+1300. The examples of the present invention in this range exhibit 0.2% yield strength at room temperature having 240 MPa or more and elongation at liquid nitrogen temperature having 30 percent or more and are excellent in ductility. Further, the examples have the relative magnetic permeability having 1.10 or less, and are suitable as a member for an electromagnet.

Industrial applicability

[0052] It is possible to obtain austenitic stainless cast steel excellent in ductility at liquid nitrogen temperature and high in yield strength at room temperature without performing solution heat treatment, and therefore, the cost of manufacturing of castings is far reduced with a high size precision of the castings. 

What we claim are:
 1. A non-magnetic austenitic stainless cast steel comprising (by mass %) 0.08 percent or less carbon, 0.1 to 1.5 percent silicon, 0.1 to 1.5 percent manganese, 13 to 15 percent nickel, 18 to 19 percent chromium, 2 to 2.5 percent molybdenum, 0.005 to 0.1 percent aluminum, and 0.12 to 0.2 percent nitrogen, the balance being iron and incidental impurities, and having an elongation at liquid nitrogen temperature having 30 percent or more, 0.2% yield strength at room temperature having 240 MPa or more.
 2. The non-magnetic austenitic stainless cast steel of claim 1, wherein the steel comprises a relative magnetic permeability having 1.10 or less.
 3. The non-magnetic austenitic stainless cast steel of claim 1, wherein the steel is obtained by being heated at temperature T(° C.) ranging from 300° C. to 550° C. and defined by the equations (1) and (2) described below for two hours or more and subsequently cooled to below 200° C. by air-cooling or slow-cooling: T≦12500[N]−1200  Eq.1T≦−5000[N]+1300  Eq.2 wherein [N] indicates the nitrogen content (%).
 4. The non-magnetic austenitic stainless cast steel of claim 1, wherein the steel is a member used below liquid nitrogen temperature.
 5. The non-magnetic austenitic stainless cast steel of claims 1, wherein the steel is a member for an electromagnet.
 6. A method of manufacturing a non-magnetic austenitic stainless cast steel, comprising: preparing a cast steel comprising (by mass %) 0.08 percent or less carbon, 0.1 to 1.5 percent silicon, 0.1 to 1.5 percent manganese, 13 to 15 percent nickel, 18 to 19 percent chromium, 2 to 2.5 percent molybdenum, 0.005 to 0.1 percent aluminum, and 0.12 to 0.2 percent nitrogen, with the remainder of the steel being iron and incidental impurities; heating the cast steel at temperature T(° C.) ranging from 300° C. to 550° C. and meeting equations (1) and (2) described below for two hours or more; and cooling the cast steel to below 200° C. by air-cooling or slow-cooling: T≦12500[N]−1200  Eq.1T≦−5000[N]+1300  Eq.2 wherein [N] indicates the nitrogen content (%). 